Resistance element



Oct. 3, 1933.

G. F, BENKELMAN RESISTANCE ELEMENT Filed April 21, 1930 I NVENTOR EWE/wwE l ATTORNEYS UNITED STATES PATENT, OFFICE 1,929,396 EEsrs'rAncE ELEMENTGlen F. Benkelman, Cleveland, Ohio, assignor to Continental Carbon,Inc., Cleveland, Ohio, a

corporation of Ohio Application April 21, 1930. Serial No. 445,851

19 Claims.

This invention relates to resistance elements and has special referenceto those of the carbon-rod type such as are often used in radioapparatus, telephone apparatus, photophone apparatus, televisionapparatus, and measuring apparatus, clocks, signal systems, and Whereveran inexpensive, reliable, resistance is desired.

These resistance elemgnjsare..enerallyxglade in t erformmog a rodorhbar, and consist of scTitfnr f'nmwnely divided carbon, (e. g. coke,graphite, gas-carbon, or lamp-black), mixed with a iinely dividedinsulating-material, (e. g. clay, sand, mica, cement, crushed-rockfetc),and a suitable inert and non-conducting binder '(e. g. bakelite,shellac, pitch, tar, rosin). The whole is baked at some temperature (e.g. between 250 Fahrenheit and 500 Fahrenheit) until it becomes sucientlyhard and the proper resistance value is attained. This resistancedepends upon three factors, any one of which can be varied, namely: theproportion of carbon to inert material, the pressure used in compactingthe mass, and the temperature (and time) employed in baking. Asindicating the importance of an accurate adjustment of proportions, 3%of carbon with 97% of insulating material may produce a device havingone million ohms resistance; 4% of carbon with 96% of insulatingmaterial may give one hundred thousand ohms; 5% of carbon with 95% ofinsulating material may give ten thousand ohms; and 10% of carbon With90% of insulating material one thousand ohms. These iigures areapproximate only and the result depends also upon the neness of thecarbon and of the insulating material and the nature of the binder. Byvarying the pressure and controlling the temperature and time of bakingit becomes possible to produce elements of accurately determinedresistance. The longer and hotter the baking the lower the resistancebecomes, and inasmuch as the effect of an electric current flowing in aresistance is to generate heat, these carbon resistance devices haveheretofore tended in actual use to become progressively less resistant.The

' foregoing is common knowledge and practice in the art. y(

I have discovered that by treating these' resistance elements withcertain materials or classes of materials this final change inresistance during use can be mostly or Wholly prevented and theresistance rendered permanent in amount so long as used within its ratedcapacity.

This material I herein term a xer. It consists of a resin having theability to tend to dissolve the binder employed, but is applied underconditions which present its dissolving that binder already in place,but which serve to deposit it in the pores of the resistance-elementready to dissolve or attack that binder whenever as a result of heatingits established condition shall tend to vary. The word dissolve as usedhere signies some form of penetration from free miscibility on the onehand to producing merely a swelling such as takes place when agar, sagoor tapioca is soaked in Water. This resin is most easily employed aftersolution in a solvent or diluent of some kind which does not dissolvethe binder already in place. Some examples of such a solvent or diluentare bayberry-Wax, carnaubawax, candelilla-wax, Montan-wax,parafiine-wax, beeswax, talloW, linseed--oil, tung-oil, pine-oil,turpentine, rosin, stearic acid, Japan-wax, or other fatty-acid orfatty-acid-ester. These materials may be of mineral, vegetable or animalorigin as indicated by the above mentioned examples. Other examples ofmaterials of mineral origin which I have found highly satisfactory inuse are what are known commercially as halowax (a chlorinatednaphthalene) and aroclor wax (a chlorinated diphenyl), both of which arecoal tar derivatives. These materials are employed in melted ordissolved form, after the element has been originally formed and baked,care being only exercised not to employ the fixer in such form orconcentration or for such length of time as to redissolve the originalbinder.

As an example of my improvements: a resistance element is made of amixture of 5% gra nite, 90% of moming sand and 15% ormlac. heSeIWground,lated, and mixed and allowed to cool into a lumpy condition, after whichthe mixture is crushed, moistened with alcohol, and compressed in diesto the des'red form and size. The resulting rods or bars are then bakedfor such a time and at such a temperature as to afford the desiredresistance per unit, after which suitable terminals are attached as bywinding wires about the ends of the rods, soldering wires or caps to theends of the rods, or by electro-plating the ends of the rods, or suchother expedient as may be adopted for the purpose. The rods may then bedipped brieiiy into a molten mixture of rosin 25 lbs. and bayberry-wax100 lbs., the immersion lasting from around one-half minute in the caseof a small resistance up to several minutes in the case of a large rod.The elements are then allowed to cool and dry, after which they can, ifdesired, be painted, lacquered, enameled, or otherwise coated to protectthem from climatic or accidental injury. Such a 'rod Will maintain itsrated resistance almost indefinitely when employed Within its ratedcapacity. Furthermore such a resistance Will not absorb moisture like anuntreated resistor and so vary in resistance With climatic conditions,but is substantially immune to atmospheric variations even though usedWithout coating with paint, enamel, or the like.

The flxers above named can be used in a great many other forms orcombinations, for example:

made to employ rosin alone without admlxture, the resistance elementwould probably be destroyed completely, either by actual disintegrationor certainly by an indefinite increase in the resistance, but byemploying the rosin in admixture with one of these diluents theimmediate disintegration of the article is prevented while a conditionfs created which renders the device permanent in character. In theabsence of such a fixer as I have described, the effect of localizedinternal heating by the electric current is gradually to decrease theresistance, probably as a result of progressive decomposition of thebinder, whereas with thls fixer present no added decomposition occursuntil the fixer has been completely baked out which under ordinaryoperating conditions can hardly occur. The proportions of resin anddiluent can be varied and different effects can thereby be produced. Ifthe proportion of resn be increased sufciently it is possible to producea device which even tends to increase gradually in resistance With use,and the proportion of resin may also be varied With reference to thetendency of the original binder to become dissolved thereby. Thus with abinder of hard bakelite the percentage of resin can desirably beincreased.

By resin I means to designate a fatty acid or an ester of a fatty acid,but I preferably employ the cheapest and commonest of its class, namelyrosin because of its ready availability. The reason I preferably employbayberry-wax as a diluent therefor is because the composition melts at aconvenient temperature producing a mixture which is sufficiently fiuidto readly penetrate the bars or rods, and that it also dries and hardensreadily forming an article which is dry and pleasant to handle and whichreadily accepts a coat of paint or lacquer. Other diluents likeparaffine will operate successfully in most respects exl cepting thatpaint or lacquer will not satisfacpart of this invention I have showncertain forms of resistors to which my invention is applicable. Fig. lis a side elevation of a resistor and Fig. 2 is a smilar view of amodified form.

Each of these resistors comprises a rod or bar 1 of the compositiondescribed, having suitable terminals attached to the end which in theform shown in Fig. 1 consists of wires 2 tightly Wound about the end ofthe rod; and in Fig. 2 consists of metal end caps 3 suitably applied andsecured thereto. The rods vary in size from one inch or less to one footor more in length and can be made with a Very wide range of resistanceand carrying capacities, although this type of device is most valuableunder conditions Where rather high resistances are desired and thecurrents employed are rather small.

However, I do not limit myself to any of the details herein describedexcept as the same are specifically set forth in my several claims whichI desire may be construed severally, each independently of limitationscontained in other claims.

Having thus described my invention what I claim is:

1. The process of stabilizing a resistance element consisting of\carbon, an insulating filler and a carbonaceous bi'drbaked together,which contains the step of impregnating said element after baking with aresin in a fatty acid solution which does not appreciably dissolve thebinder When cold but begins to do so at the established operatingtemperature of the said element.

2. The process of stabilizing a resistance element consisting of carbon,an insulating filler and a carbonaceous binder baked together, whichcontains the step of impregnating said element after baking with amolten solution of a resin in an organic solvent which does notappreciably dissolve the binder when cold but begins to do so at theestablished operating temperature of said element.

3. As an article of manufacture and sale, a resistance elementconsisting of a baked mixture of finely divided conducting andnon-conducting particles imbedded in a carbonaceous binder, the wholebaked mixture being impregnated with a solution of a vegetable resin inan organic solvent said solution being inert to the binder when cold butwhich begins to dissolve the same when heated to a temperature at theestablished operating temperature of said element.

4. As an article of manufacture and sale, a resistance elementconsisting of a baked mixture of conducting and non-conducting materialsimbedded in a carbonaceous binder, the Whole baked mixture beingimpregnated with a solution of a vegetable Wax in a vehicle ofbayberry-Wax in which the said Wax does not appreciably dissolve thebinder when cold but tends to do so upon a predetermined rise intemperature of the vehicle.

5. The process of making permanent resistance elements which containsthe steps of rst mixing together an electrically conducting material,

a non-conducting material, and a carbonaceous binder, compressing themass into elements of desired size and shape, heating the same until thebinder takes on a degree of set, and finally impregnating the pores ofthe device with a carbon compound which is inert to the binder when coldbut has the property of tending to dissolve the binder when heated to atemperature at and above the established operating temperature of thedevice.

6. The process of stabilizing a resistance element consisting of amixture of conducting and non-conducting materials imbedded and baked ina carbonaceous binder, Which contains the step of impregnating saidelement after baking with a vegetable resin which does not appreciablydissolve the binder when cold but tends to do so when hot.

7. The process of stabilizing a resistance element consisting of amixture of conducting and non-conducting materials imbedded and baked ina carbonaceous binder, and which includes the step of Wholelyimpregnating the mixture after baking With a solution of a vegetable waxin a vehicle, in which the said Wax does not appreciably dissolve thebinder when cold but tends to do so at and above the establishedoperating temperature of the element.

8. The process of stabilizing a resistance element including carbon, aninsulating filler, and a carbonaceous binder baked together, whichcontains the step of impregnating said element after baking with amolten solution of a carbon compound in an organic solvent which issolid at normal temperatures and tends to dissolve the binder at andabove established operating temperatures of said element.

9. The process of stabilizing a resistance element, including carbon, aninsulating ller, and a carbonaceous binder baked together, whichcontains the step of expelling the air from Within said element bypermeating the voids naturally occurring therein with a molten solutionof a carbon compound in an organic solvent, which is solid at normaltemperatures and which begins to dissolve the binder at an establishedoperating temperature.

10. The process of stabilizing a resistance element, including carbon,an insulating ller, and a carbonaceous binder baked together, whichcontains the step of permeating and lling the voids of said element witha coal tar derivative which is solid at normal temperatures and beginsto dissolve the binder at operating temperatures.

11. The process of stabilizing a resistance element, including carbon,an insulating filler, and a carbonaceous binder baked together, whichcontains the step of expelling the air from Within said element bypermeating the voids naturally occurring therein with a molten solutionof a substance of vegetable origin which is solid at normal temperaturesand tends to dissolve the binder at and above established operatingtemperatures.

12. The process of stabilizing a resistance element consisting ofcarbon, an insulating filler, and a carbonaceous binder baked together,which contains the step of impregnating said element after baking, Witha molten solution of a ller including vegetable Wax in an organicsolvent which is solid at normal temperatures and tends to dissolve thebinder at and above the established operating temperatures of theelement.

13. The process of stabilizing a resistance element consisting ofcarbon, an insulating filler, and a carbonaceous binder baked together,which contains the step of impregnating said element after baking with aresin in a solution of an ester of a fatty acid which solution does notappreciably dissolve the binder when cold but which tends to do so abovethe established operating temperature of the said element.

14. The process of stabilizing a resistance element including carbon, aninsulating ller, and a carbonaceous binder baked together, whichcontains the step of impregnating said element after baking with amolten solution of a carbon compound in an organic solvent Which issolid at normal temperatures and tends to dissolve the binder above theestablished operating temperature of said element, whereby thelocalization of heat in portions of said element is prevented.

15. The process of stabilizing a resistance element including carbon, aninsulating-filler, and a carbonaceous binder baked together, Whichcontains the step of expelling the air from Within said element bypermeating the voids naturally occurring therein With a molten solutionof a substance of vegetable origin which is solid at normal temperaturesbut which begins to penetrate the binder at and above establishedoperating temperatures of the element whereby the localization of heatat portions of said element is prevented.

16. As an article of manufacture and sale, a resistance elementconsisting of a baked mixture of nely divided conducting andnon-conducting particles imbedded in a carbonaceous binder, the Wholebaked mixture being impregnated with a solution of a resin in an organicsolvent said solution being inert to the binder when cold but whichbegins to dissolve the same when heated to a temperature at theestablished operating temperature of said element.

17. As an article of manufacture, a resistance element including a bakedmixture of conducting and non-conducting material imbedded in acarbonaceous binder, the Whole baked mixture being impregnated with amolten solution of a carbon compound in an organic solvent which issolid at normal temperatures but which tends to dissolve the binder atand above the established operating temperatures of said element.

18. As an article of manufacture, a resistance element including a bakedmixture of conducting and non-conducting material imbedded in acarbonaceous binder, the Whole baked mixture being impregnated with amolten solution of a carbon compound in an organic solvent, which issolid at normal temperatures and which begins to dissolve the binder atestablished operating temperatures of the element.

19. As an article of manufacture, a resistance element including a bakedvmixture of conducting and non-conducting material imbedded in acarbonaceous binder, the whole baked mixture being impregnated with acoal tar derivative which is solid at normal temperatures and begins todissolve the binder at operating temperatures.

GLEN F. BENKELMAN.

